Method of making low-alloy steel



Patented Feb. 12, 1952 METHOD OF MAKING LOW-ALLOY STEEL- Maurice J. Day, Oak Park, 111., and Gerald D. Rahrer, Gary, Ind., assignors to United States Steel Company, a corporation of New Jersey No Drawing. Application August 6, 1948,

. Serial No. 43,012

1 Claim. '1

This invention relates to the manufacture of carburized articles and, in particular, to lowalloy steels containing boron suitable for such articles.

Machine parts subject to wear should have a hard surface and a tough ductile core. This result has been accomplished heretofore by carburizing and heat-treating. At first, plain carbon steel was utilized but it was necessary to restrict the carbon content to about 25% maximum to avoid excessive hardening and embrittlement of the core. Such low-carbon steel lacks sufiicient strength for certain applications, however, so it became common to add alloying elements, preferably nickel, either alone or in combination with molybdenum and other elements. Substantial amounts thereof (in excess of 3%) are necessary for the increased strength desired in the core, and the cost of the added elements is substantial.

A further objection to the use of higher alloy concentrations is the reduction in surface hardness after carburizing, which results from a combination of absorbed carbon with the increased content of alloying elements, apparently leading to the retention of austenite in the case, or surface layers, after quenching. Any reduction in surface hardness, of course, defeats the whole purpose of heat-treatment, since the highest possible hardness on the outside is desirable for satisfactory tools. The composition of steel for carburizing, therefore, has been a compromise between the desired surface hardness and the necessary strength of the interior of the part.

The presence of retained austenite in the carburized case of a machine part also impairs the properties and serviceability of the part in other respects. The austenite very often transforms to the alpha state while the part is being used. The resultant hard constituents occupy a greater space than the original austenite, and this expansion increases compressive stresses in localized areas. The stresses so set up may strain the surface layers of the steel sufficiently to break them away from the interior portion of the part. In machines in which the dimensions of the part in question are restricted by contiguous parts, the operation of the machine as a whole will be impaired in many cases by an increase in the dimensions of one part caused by the expansion described above.

We have invented a novel method for making steel articles having a high strength and ductility of the core and a high surface hardness characterized by alloying low carbon carburizing grades of steel with small percentages of boron, carburizing their surface, and then quenching the carburized articles from a temperature chosen to give the optimum properties of the core. Specifically, we provide a steel having a minute quantity of boron in addition to other alloying elements in reduced amounts compared to previous practice.

We are aware that boron has been added to steel heretofore to increase the hardness penetration, namely, thedepth below the surface of an article throughout which the hardness, much lower than the maximum potential hardness, is above a predetermined minimum. Our invention, however, is based on the discovery that a small addition of boron exerts an unexpected effect on steel which is to be carburized, viz., it increases the cooling rate necessary in quenching to obtain optimum hardness in the carburized zone without undesirably affecting the hardenability of uncarburized portions remote from the surface. In other words, we have found that boron added to steel intended for carburizing prevents the retention of residual austenite in the surface layers having their carbon content increased by its absorption into the solid metal.

A maximum hardness of alloy steels carburized to a high carbon content may be obtained by reducing the cooling rate in quenching, but this lowers undesirably the hardness penetration into the uncarburized portion. The steels of our inven tion make it possible to maintain the cooling rate as high as needed to achieve the optimum properties of the core without objectionably decreasing the maximum hardness of the surface. We are thus able to utilize the full hardenability of any steel subject to impairment of its transformation characteristics by high carbon concentration.

In a preferred practice, we add to molten lowcarbon carburizing steels sufficient boron-containing substance such as ferro-boron or complex boron alloys to give a boron content of from .0005% to .004% in the finished steel. Speaking generally, the steels of our invention comprise carburizing grades of alloy steels with a carbon content of less than 25%. The concentration of their hardening elements may be reduced as taught in the Crafts Patent No. 2,280,283. These steels, when carburized and quenched in water from 1550" R, exhibit a hardness above on the Rockwell 0 scale. The addition of boron may 3 be made in the ladle or in the ingot molds. The molten steel is cast into ingots and the ingots are reduced in steps to stock of the desired final shape and size by known rolling operations.

For purposes of illustration, two examples of typical practice of our invention are herebelow designated A and B, respectively. For comparison, a known carburizing steel having the same hardenability but containing no boron (AISI A-4815) was similarly treated and tested, this sample and its characteristics being designated C in the following tables. All samples were rolled to the same size and heat-treated and quenched alike. The composition of the steel of samples A, B and C is:

TABLE I Specimen Mn S P Si Ni Cr Mo 33 A ll 53 032 011 .28 1.77 .09 .26 .0012 B 1S 87 .036 .018 40 .55 .54 .23 .0045 C .16 56 020 .020 .28 3. 53 .09 24 Ni] TABLE H Rockwell C hardness values at various distances from quenched end Speci- A i 37. 3'7. 5 36 35 30 22 19. 5 16 15. o

The above demonstrates that the hardening characteristics of the three steels in their original compositional state are quite similar.

To show the hardness characteristics of the above steels in the carburized condition, specimens representing the three steels, the analyses of which are given in Table I, were forged from the rolled section to 1%." rounds, normalized at 17 00 F., machined to 1" rounds, 4" long and carburized in the same carburizing compound. One specimen of each steel was carburized for eight hours, another for twelve hours and another for sixteen hours. The specimens were cooled in the carburizing box, then heated to 1550 F. and quenched in water. The heating prior to quenching was conducted under conditions which reduced oxidation to a minimum and permitted hardness determination on the surface without any cleaning. The hardness characteristics of the carburized quenched steels are given in Table III. a

TABLE III Hardness as quenched-Rockwell "C Garburized Carburized Carburlzed Speclmen 8 hours 12 hours 16 hours A comparison of the hardness values given in Table III clearly indicates that samples A and B of steels containing boron have a higher hardness at the surface of the carburized case after quenching than sample C containing no boron. This appears to be the result of a reduction of the retained austenite content in the carburized case of samples A and B. The ability to harden in the original composition, and hence in the interior portions of a machine part is of the same order of magnitude in the steels with boron addition represented by samples A and B as in a steel without boron represented by sample C, as shown in Table II. The combination of properties of the boron steels, represented by samples A and B, therefore, is an improvement over the properties of the boron-free steel, represented by sample C and the invention thus represents a marked advance over prior methods of making steel for carburizing.

The lower hardness of steel C on the carburized surface is caused by the retention of austenite, a direct result of the sluggish transformation from austenite in high-carbon alloy steels on cooling to transformation temperatures. In such steels, higher hardness may be obtained in the carburized portions by reducing the cooling rate, but this produces an undesirable effect on the uncarburized portion, constituting the core and revealed in its decreasing hardenability after quenching. The addition of boron acts as if it increases the maximum cooling rate which will achieve the optimum hardness on the carburiz ed surface layers without an undesirable effect on the uncarburized layers in the interior of the steel mass.

The addition of boron permits the utilization of the maximum potential hardness of any hardenable steel having its transformation characteristics impaired by a high carbon concentration, while at the same time it makes possible a desirable reduction of the concentrations of alloying elements in steels intended for carburizing.

Although we have disclosed herein only a preferred practice of the invention, it will be recognized that changes in the procedure disclosed may be made without departing from the spirit of the invention or the scope of the appended claim.

We claim:

In a method of modifying the response to carburizing and quenching of hardenable steels which tend to retain austenite in the surface oi. an article composed thereof, after carburizing and quenching, and thus limit the surface hardness thereby obtainable, the steps of making a heat of steel containing from .11 to .18% carbon, from .53 to .87% manganese, from .55 to 1.77% nickel, from .09 to 54% chromium, and about 25% molybdenum, the balance being substantially iron with the residual elements common to such steel, adding boron to said heat while molten, in an amount from .0005 to 004%, and carburizing articles formed from said steel to increase the carbon content of the surface layer of the articles, thereby reducing the amount of austenite retained in said layer after quenching and increasing the hardness obtainable on the surface thereof.

MAURICE J. DAY. GERALD D. RAHRER.

(References on following page) REFERENCES CITED UNITED STATES PATENTS III 6 OTHER REFERENCES Report on Special Alloy Addition Agents, pages 4 and 5. Published in 1942 by The American Iron and Steel Institute.

The Iron Age, March 25, 1943, pages 45 to 50.

Number N Date Metal Treatment, publication for the Spring 2,280,283 Crafts Apr. 21, 1942 quar er, 1947, pages 5, and 11 to 13. Published ,283,299 Tisdale May 19, 1942 by Industrial Newspapers, Limited, London, 2,478,420 Payson Aug. 9, 1949 10 England. 

